10th January 2015

Kepler telescope confirms 1000th exoplanet

NASA has announced the 1000th confirmed exoplanet discovered by the Kepler Space Telescope. Three of the newly confirmed exoplanets were found to orbit within habitable zones of their parent stars.

Launched by NASA in 2009, the Kepler space telescope became the first instrument capable of finding Earth-sized and smaller extrasolar planets. Originally the mission was expected to last until 2016, but the second of four reaction wheels (used for aligning the telescope) failed in May 2013 – disabling the spacecraft and putting its future in doubt.

However, an alternative plan named K2 "Second Light" was presented for consideration in November 2013. This would involve Kepler operating in a reduced capability mode, but able to continue exoplanet discovery, using an ingenious "virtual" reaction wheel. K2 began in May 2014 and had scanned 35,000 stars by the end of the year. Its first confirmed exoplanet was detected in December 2014, a hot super-Earth 180 light-years away in the constellation Pisces.

Combining Kepler's original tally with hundreds of results from K2, a total of 4,175 potential candidates have now been observed, the 1,000th of which was officially verified this week, after a further eight new planets were added to the "confirmed" list. Three of the newly-validated worlds are located in their suns' habitable zone – the range of distances from the host star where liquid water might exist on the surface of an orbiting planet. Of the three, two are likely to be made of rock, like Earth.

Kepler-438b and Kepler-442b, pictured here in the top row, are less than 1.5 times the diameter of Earth. Kepler-438b lies around 475 light-years away and is 12% bigger than Earth, while Kepler-442b is 1,100 light-years away and 33% bigger than Earth. Both have relatively short orbits of 35 days and 112 days – similar to that of Mercury (88 days) in our own Solar System. However, their parent stars are smaller and cooler than Sol, making their habitable zones closer. The third, Kepler 440b, is less likely to be a rocky planet. On the bottom row are small habitable zone planets confirmed in previous years including Kepler-186f, Kepler-62e and Kepler-62f.

“Each result from the planet-hunting Kepler mission's treasure trove of data takes us another step closer to answering the question of whether we are alone in the Universe,” said John Grunsfeld, associate administrator of NASA’s Science Mission Directorate at the agency’s headquarters in Washington. “The Kepler team and its science community continue to produce impressive results with the data from this venerable explorer.”

“With each new discovery of these small, possibly rocky worlds, our confidence strengthens in the determination of the true frequency of planets like Earth,” said co-author Doug Caldwell, SETI Institute Kepler scientist at NASA's Ames Research Centre in California. “The day is on the horizon when we’ll know how common temperate, rocky planets like Earth are.”

“Kepler collected data for four years – long enough that we can now tease out the Earth-size candidates in one Earth-year orbits,” said Fergal Mullally, another SETI Institute Kepler scientist. “We’re closer than we’ve ever been to finding Earth twins around other sun-like stars. These are the planets we’re looking for.”

Many of these exoplanets will be targeted in follow-up studies by the James Webb Space Telescope to characterise the atmospheres of distant worlds and search for signatures of life. Kepler has now discovered 1,000 exoplanets, but the James Webb and other future telescopes are expected to find even more.

If the current rate of discovery continues, then the number of confirmed exoplanets is predicted to surpass 13 million by the 2050s. This could reach into the hundreds of billions by the end of the century. In other words, pretty much every world in the Milky Way will have been catalogued. Assuming our telescopes also increase in magnification, we should also be able to observe them in close-up detail. We will then have to look at neighbouring galaxies for new planets.

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